1. Field of the Invention
This invention relates to electrophoretic liquid development and more particularly to an improved process of plural stage development.
2. Description Relative to the Prior Art
In the liquid development of electrostatically charged latent images, as in electrophotography and in other processes that form and develop electrostatic charge patterns, a substrate having a charge pattern on its surface is contacted with a liquid developer which is essentially a suspension of colloidal toner particles in an insulating liquid. Liquid developers normally also contain a stabilizer or charge control agent. The latter is an ionic compound which controls the magnitude of the charge on the toner particles and aids in maintaining a stable charge on the toner particles within the insulating carrier liquid.
Liquid developers can be used in single stage or plural stage development processes. Examples of the latter may include the sequential development on a photoconductor of two or more color-separation images, the annotation of a previously developed image, or the repeated re-exposure and development of images on a reusable photoconductor, with transfer of images upon completion of a number of imaging cycles.
In certain plural stage development processes which use liquid developers, a problem has been found which is especially significant in processes for the electrophotographic reproduction of multi-color images of graphic arts quality. In these processes electrostatic latent images are formed sequentially on a chargeable substrate such as an electrophotographic medium, with liquid development or toning of each latent image before the next image is formed. A leading example of this type of system involves processing an electrophotographic medium sequentially through a series of four imaging cycles including four sequential development or toning stages.
While the present invention is useful in any electrostatic imaging process wherein a charge pattern is formed and developed with a liquid developer on a surface which has previously been developed with a liquid developer, it is particularly useful in combination with a recently developed electrophotographic process of making lithographic color proofs, such as described in U.S. Pat. No. 4,600,669. In that process a photoconductor, which has a uniformly charged thin transparent dielectric overlayer, is subjected to a series of exposures through registered color separation transparencies. After each exposure the dielectric layer is developed with a liquid developer, and the surface is again uniformly charged, exposed, and developed. The sequence is repeated for each of the color transparencies, usually four. It has been found that image defects occur in areas of the image which are toned areas of a previous imaging cycle. The defect appears as a color contamination of the previously toned areas which imparts a non-uniform density, thereby altering color rendition, a defect which, though perhaps acceptable in some kinds of add-on, plural stage imaging, is not acceptable for producing high quality images as required, for example, in the graphic arts field.
It has been discovered that the described image defects in plural-stage liquid development appear to be caused by the presence of forces, substances, or charge acceptance in the previously toned areas that interfere with the correct subsequent development in those areas. Although the nature of those forces or substances is not clear, the present invention provides apparatus that reduces or eliminates the problem.
U.S. Pat. No. 4,660,503 describes the improvement in the process of the '669 patent of cleaning the image by brushing the surface of the toned image with a soft, smooth, and supple fibrous brush member between the development of an image and the exposure of the next image to remove counter-ion material from the untoned areas which interferes with subsequent toning cycles in the above-mentioned process. The improvement of the '503 patent affects only the untoned areas of the developed substrate or photoconductor and has been found to have no effect on the toned areas where the present problem has been found.
Thus, the problem addressed by the present invention is that of preventing color contamination of previously toned areas rather than preventing contamination of the untoned areas. The solution must not adversely affect the desired electrophoretic deposition of toner particles in both previously toned and untoned areas to develop the latent electrostatic image areas of subsequent imaging cycles. The above-referenced, previously filed co-pending applications are directed to a method and apparatus for preventing the contamination of previously toned areas by physically smoothing the toned image before generating the subsequent image, such as by buffing with a rotating brush.
Accordingly, the provision of an additional simple and relatively easily implemented solution to this problem, particularly if it can be done at a nominal cost and does not have any significant negative effects on the resulting image, would be very advantageous in providing a variety of solutions from which to choose.